Riflescope mount

ABSTRACT

This invention relates to an improved riflescope mount, and, more specifically, to the novel detachable connection between the front ring and base. The ring has the underside thereof provided with a downwardly tapered frusto-conical self-centering post terminating at its apex in a short cylindrical section having springable tangs projecting both forwardly and rearwardly therefrom to define a clip. The base, in turn, has a complementary, though relatively shallower, frusto-conical seat adapted to mate with the post and an oversize transversely extending slot intersecting said seat sized to loosely receive the clip when aligned therewith. The clip and underside of the base bordering the slot therein are so shaped and dimensioned that upon rotation of the ring through an angle of 90*, the tangs will spring down enough to pass into position underneath the dimples produced by the seat thereabove while drawing the post snugly down atop the latter.

ilnited States Patent [1 1 Burris [451 Apr. 29, 1975 RIFLESCOPE MOUNT [75] Inventor: Donald J. Burris, Greeley, C010. [73] Assignee: Burris Company, Greeley, Colo. [22] Filed: July 9, 1973 [21] Appl. No.: 377,744

Primary Examiner-James C. Mitchell Attorney, Agent, or Firm-Edwin L. Spangler, Jr.

[57] ABSTRACT This invention relates to an improved riflescope mount, and, more specifically, to the novel detachable connection between the front ring and base. The ring has the underside thereof provided with a downwardly tapered frusto-conical self-centering post terminating at its apex in a short cylindrical section having springable tangs projecting both forwardly and rearwardly therefrom to define a clip. The base, in turn, has a complementary, though relatively shallower, frustoconical seat adapted to mate with the post and an oversize transversely extending slot intersecting said seat sized to loosely receive the clip when aligned therewith. The clip and underside of the base bordering the slot therein are so shaped and dimensioned that upon rotation of the ring through an angle of 90, the tangs will spring down enough to pass into position underneath the dimples produced by the seat thereabove while drawing the post snugly down atop the latter.

18 Claims, 10 Drawing Figures RIFLESCOPE MOUNT Riflescope mounts have been used for many years as the brackets." so to speak. that rigidly fasten the riflescope to the barrel of a rifle or sometimes a pistol or shotgun. These mounts consist of front and rear split rings and a base. the latter coming in one or two parts depending on the type of gun upon which it is to be fastened or the preference of the shooter. The rear ring is transversely adjustable relative to the base so as to introduce those windage corrections necessary to com pensate for misalignment occasioned by improperly located base-mounting holes in the barrel. The front ring. on the other hand. occupies a fixed position relative to the base although it is detachably connected thereto.

Some shooters like to use a scope on their riffle for some types of shooting say. for example. varmint hunting: yet. prefer open sights at other times like target shooting or big game hunting. Whatever the reason. occasions arise not infrequently when the scope together with the mounting rings therefor are detached from the base. The problem in the past has always been to return the scope to its so-called zero" position repeatedly and reliably.

Generally speaking. the prior art riflescope mounts have provided a satisfactory solution to the problem of returning the adjustable rear ring to the exact same po sition it formerly occupied but such structures have not fared so well in doing the same for the fixed front ring. The main difficulty seems to be that of wear on the mating surfaces which. eventually. results in a loose fit that is intolerable for accurate shooting. Also. once the connection comes loose. the prior art mounts make no provision for tightening same and it must be replaced.

It has now been found in accordance with the teaching of the instant invention that these and other shortcomings ofthe prior art riflescope mounts can. in large measure. be eliminated by the simple. yet unobvious. expedient of biasing a frusto-conical post on the underside ofthe front fixed ring down snugly into a relatively shallower complementary frusto-conical seat in the base by turning the springable tangs at the apex of the post until they spring down and under that portion of the base bordering the clip-receiving slot therein. In the preferred emobidment of the invention. the tolerances between the mating surfaces of the clip and base preparatory to their moving into assembled relation are on the order of i 0.001 inches; therefore. they can be made planar and require no special edge treatment. On the other hand. alternative embodiments of the inven* tion requiring less in the way of close tolerances use cam surfaces to insure that the leading edges of the tongue-forming tangs will pass freely over the margins of the clip-receiving opening in the base without interfering therewith. In either event. once the ring has been turned such that its clip is no longer aligned with. but intersects. the clip receiving slot in the base. substantial planar areas normal to the axis of relative rotation mate with one another in face-to face contacting relation. These mating planar surfaces together with mating complementary frusto-conical surfaces cooperate with one another to insure that the front ring will repeatedly and reliably return to the position it formerly occupied insofar as its elevation above the barrel and the axis of rotational movement thereof. While the ring can be removed and replaced in the base or swing from side-toside relative to the latter along with a scope to accommodate lateral shifts in the rear ring without producing any appreciable wear in the mating surfaces. if such should occur. it becomes a simple matter to bend the tangs upwardly toward the ring again thus restoring their original biasing ability.

It is. therefore. the principal object of the present invention to provide a novel and improved riflescope mount.

A second objective is the provision of a device of the class aforementioned wherein the front ring remains fully adjustable from side-to-s'ide about a vertical axis. yet. said axis remains in essentially the same position relative to the gun barrel even though the ring is demounted from the base and remounted thereon over and over again.

Another object of the within described invention is to provide a front riflescope mounting ring and base therefor that uses springable tangs acting against surfaces of the base bordering a slot therein to bias :1 pair of complementary frustoconical surfaces into coaxial relation seated one within the other.

Still another objective of the invention forming the subject matter hereof is the provision of a front scope mounting ring and base for use therewith that can either be separate or a part of a common base carrying the adjustable front ring.

An additional object of the invention herein disclosed and claimed is to provide a front scope mounting ring having a pair of springable tangs that can be reset to compensate for any normal wear in the mating surfaces.

Further objects are to provide a riflescope mount and. more specifically. a front ring and base subassembly therefor that is rugged. simple. easy to use. inexpensive to make. versatile. compact. decorative and readily adapted for use with various types. styles. sizes. makes and designs of both scopes and firearms.

Other objects will be in part apparent and in part pointed out specifically hereinafter in connection with the description of the drawings that follows and in which:

FIG. 1 is a perspective view looking down upon the complete scope mount from a vantage point above and slightly to the rear of its right side. the riflescope having been indicated by broken lines;

FIG. 2 is a fragmentary exploded perspective view to a larger scale than FIG. 1 and showing only the front ring preparatory to its being inserted and rotated in its seat in the base. all of the latter being viewed as it would be seen from a point above the front right corner thereof;

FIG. 3 is a still further enlarged view of the front ring and associated seat therefor in the base. half of said ring having been shown in section and the other half in front elevation;

FIG. 4 is a fragmentary section to the same scale as FIG. 3 taken along line 4-4- thereof;

FIG. 5 is a fragmentary view to the same scale as FIG. 3 and 4 showing the right half of the ring in side elevation while the left half along with the base is pictured in section;

FIG. 6 is a view to the same scale as FIGS. 3.4 and 5 showing the preferred clip from above. partly in plan and partly in section;

FIG. 7 is a view much like FIG. 6 except that it shows a slightly modified form of clip having beveled corners at its diametrically located extremities that define leadin cams;

FIG. 8 is a fragmentary bottom plan view of the preferred form of seat in the base for the front ring;

FIG. 9 is a fragmentary bottom plan view like FIG. 8 except that it shows a modified form of seat with diametrically located oppositely inclined cam surfaces emerging at their terminal ends onto coplanar shelves: and,

FIG. 10 is a section taken along line 10-10 of FIG. 9.

Referring next to the drawings for a detailed description of the present invention and. initially. to FIGS. I and 2 for this purpose, reference numeral I0 has been chosen to designate the riflescope mount in its entirety while numeral 12 similarly refers to the base and numeral l4 to the front ring detachably connected to the latter. Rear ring 16 is of conventional design as is its seat (not shown) in the base I2. The base may com prise a unitary structure as shown with a bridgeforming element 18 interconnecting the front and rear platforms 20 and 22, respectively. of the base atop which the rings sit. or alternatively, these platforms may be separate entities and the bridge-forming element therebetween eliminated. Both constructions are old in the art and form no part of the present invention.

The portions of both the front and rear rings visible above the base are not only identical, but of conventional design as well. They each include a short upright post-forming element 24 topped by the integrally formed lower half 26 of split retaining ring 28. The top half 30 of each ring is detachably connected to the lower half by means of threaded fasteners 32 in the usual manner. The riflescope 34 represented by dotted lines in FIG. I is thus fastened to the rifle barrel or receiver. neither of which have been illustrated. The screw 36 on the near side of the rear platform cooperates with another screw hidden on the far side thereof to mount the rear ring for limited transverse adjustment so as to correct for any misalignment of the base relative to the axis of the gun barrel occasioned by improper installation. Once again. this is an old practice in riflescope mounts and is only mentioned here for the reason that the front ring 12 must be able to rotate in its seat 38 to the extent necessary in order to accommodate the very limited swing of the scope that occurs when the rear ring 16 is shifted to one side or the other.

The significant and novel features of the instant invention are those which relate to the detachable mounting of front ring 14 in its seat 38 in the front platform of the base 12. It is these features which form the subject matter of FIGS. 2-10, inclusive. of the drawings, immediate reference being made to FIGS. 2, 3, 4 and S for a detailed description of the seat 38 and the selfcentering post 24 of the front ring 14.

The lower extremity of the self-centering post 24 of the front ring is shaped to form frusto-conical tip 40 which is downwardly and inwardly tapered in contrast with the prior art front rings which are oppositely fashioned to providea downwardly and outwardly flared tip section that is truncated vertically on both sides and seats upon rotation-through a quarter turn into a complementary downwardly and outwardly flared conical seat on the underside of the base bordering an opening therein sized to receive same. The downwardly and inwardly tapered seat 38 in the front platform 20 of the base 12 is shaped to complement the frusto-conical tip 40 on the underside of the front ring, these complementary elements when assembled into mated relation also being coaxial and having their common axis intersecting the axis of the ring thereabove at a right angle in all adjusted positions of the latter.

The narrow end or apex of frusto-conical tip 40 adjoins a short cylindrical section 42 upon the underside of which is integrally formed a clip 44 having oppositely extending springable tangs 46 that cooperate with certain surfaces 48 on the underside of the base which will be described in detail presently to firmly bias and hold the mating frusto-conical surfaces of the seat and post in annular substantially uninterrupted area contact with one another. The greater the biasing force exerted by the tangs, the tighter the frusto-conical tip 40 is drawn into its complementary seat 38. In this connection, it becomes significant to note the relationship re vealed in FIG. 5 wherein the conical surface of tip 40 continues well out beyond and above the corresponding complementary surface of the seat 38 so that planar surface 50 of the platform 20 bordering the latter always remains in spaced relation beneath shoulder 52 that adjoins said tip 40 on both sides thereof as shown.

Intersecting the seat 38 is a transverse slot 54 dimensioned to loosely receive clip 44 on the bottom of the post 24. The short cylindrical neck 42 separating the clip from the apex of the tip of the self-centering post has a diameter somewhat less than the width of slot 54 at that point so as to leave a small gap 56 therebetween as shown in both FIGS. 3 and 5. This gap is actually produced by extending the tip 40 downwardly and inwardly a distance beyond the corresponding margin of the relatively-shallower seat 38 and, in fact. it could be continued all the way to the top surface 58 of the foot thereby eliminating cylindrical neck 42 altogether. the latter having been included primarily to facilitate fabrication of the post although it does have some limited functional significance in providing a larger area of attachment between the clip and post. In any event. gap 56 insures the fact that the neck and opposed surfaces of the slot bordering the latter will not engage one another and prevent proper seating of the mating complementary frusto-conical surfaces.

Next, with particular reference to FIGS. 3, 5, 6 and 8. it will be noted that the underside of the base 12 is shaped as indicated at 60 to conform with the surface of the gun barrel atop which it is to be mounted but, in addition, it has a downwardly opening cylindrical socket 62 therein that occupies a coaxial relation to the seat 38. It is the roof-forming portion of this socket alongside transverse slot 54 that defines the tangreceiving surfaces 48 mentioned previously. The tangreceiving surfaces 48 are, in the preferred form of the invention shown in FIGS. 3, 5 and 8, coplanar and normal to the common axis of the mating frusto-conical surfaces, which is also the axis of rotation of the ring as it is rotated approximately a quarter of a turn between its seated and unseated positions. The upper surfaces 58 of the tangs 46 are also preferably coplanar with one another and, in addition. essentially coplanar with the aforementioned tang receiving surfaces 48 of the base when the mating complementary frustoconical surfaces are seated firmly one within the other as shown in FIGS. 3 and 5. By holding tolerances such as to produce an interference fit between the mating frustoconical and planar surfaces of the assembly, no cam surfaces are really needed to get the parts started into locking engagement with one another as the leading corners 64 of the tangs (assuming clockwise rotation from above) will spring down the few thousandths ofan inch necessary to accomplish the latter. In fact. after just a few actuations. these leading corners will wear off sufficiently to produce all the cam surface that is necessary to get the tangs past the margins of the slot so that the opposed planar surfaces can move into sliding faceto-face engagement with one another.

Once rotated a quarter of a turn into the locked posi tion shown in FIGS. 3. 4 and 5. the tangs cooperate with the roof-forming planar surfaces 48 bordering the slot 54 in the base to firmly bias the complementary frusto-conical surfaces into tight area contact with one another; yet. the interference fit so produced is not so tight as to preclude the limited relative rotation between the front ring and base as may be required to accommodate any lateral shift in the rear ring transferred forwardly by the rigid riflescope linking them together. The mating conical surfaces. of course. return the front ring repeatedly to its same position relative to the base insofar as the elevation of its axis above the latter. Also. the front ring at least may be removed and replaced many more times than the prior art front ring mounting assemblies before it gets so worn and loose as to require replacement. ln fact, if the tangs should fail to provide the desired interference fit after a time. the ring neednt be discarded as is usually the case with the present scope mounts. but instead. the tangs need only be bent up a thousandth of an inch or so to restore their original biasing action and compensate for any wear that may have occurred.

Finally. with brief reference to FIGS. '7. 9 and 10, two of the modified forms of the unit just described will be set forth in detail. FIG. 7 is directed to a modified form of front ring wherein the base 44m has the leading corners 64121 of the tangs 46m beveled. chamfered. rounded or otherwise shaped to define lead-in cams adapted to get the cam-carrying planar face 58:11 past the edges of the slot 54 and over on to the opposed planar surfaces 48 of the base. As previously noted. the unit works quite well without such cam surfaces provided close tolerances are held. however. in the absence thereof. the cam can be. and probably should be. used. This modified clip should be used with the full planar roof-forming surfaces 48 of FIG. 8.

As a second alternative the clip of FIGS. 3, 4 and 6 can be used with a slightly modified base of H65. 9 and 10 wherein instead ofthe corners of the clip being beveled to define cam surfaces. the corners 66 of the roofforming surface 4811! first engaged by the latter are beveled or otherwise rounded off. to define the cam sur faces adapted to spring the tangs down to the minimal extent required to get the planar surface 58 of the latter over onto the opposed planar base surface 48. The same thing that was said about the previous modification, of course. applies here. namely. that if proper tolerances are held. no cam surfaces are necessary on either the base or ring or both.

What is claimed is:

1. Apparatus for detachably fastening the front end of a riflescope to a rifle so as to permit limited relative pivotal movement of the former relative to the latter which comprises: a rigid base fixedly attachable to the rifle having a downwardly and inwardly tapered seatmetrical slot intersecting said depression establishing communication between the latter and the socket therebeneath; ring means having a post depending from the underside thereof terminating at its lower end in a downwardly and inwardly tapered frusto-conical tip section complementary to the seat-forming depression in the base; and. latch-forming means shaped to enter the slot in the base when aligned therewith operative upon limited relative rotation between said base and ring to releasably lock same together in assembled relation, said means comprising a clip attached to the truncated apex of the tip section having a pair of springable tangs projecting in opposite directions from said point of attachment, said tangs being spaced beneath the frusto-conical section thereabove a distance such that they will seat against the roof-forming surface when rotated into face-to-face contacting relation therewith. and said tangs and roof-forming surface cooperating with one another in assembled relation to continually bias said opposed frusto-conical surfaces into tight slidable mated engagement.

2. The apparatus as set forth in claim 1 in which: the roof-forming surface at the top of the socket is essentially planar and perpendicular to the axis of ring rotation.

3. The apparatus as set forth in claim 1 in which: the frusto-conical tip section of the post is a right conical section having its axis passing through the center of the ring means.

4. The apparatus as set forth in claim 1 in which: the

complementary frusto-conical surfaces are both right conical sections; and. in which the tangs and roof' forming surface cooperate with one another in assembled relation to maintain a coaxial relationship between the respective axes of said complementary frustoconical surfaces.

5. The apparatus as set forth in claim 1 in which: the frusto-conical tip section is larger at its base than the complementary frusto-conical seat-forming surface so as to extend above the latter when in assembled relation and define a stop effective to keep the post from contacting the base.

6. The apparatus as set forth in claim 1 in which: the upper base engaging surface of the tangs is essentially planar and normal to the axis of ring rotation.

7. The apparatus as set forth in claim 1 in which: downwardly facing shoulders define the margin separating the frusto-conical tip section from the post thereabove; and. in which the mated frusto-conical surfaces cooperate with one another in assembled relation to hold said shoulders spaced above the top of the base.

8. The apparatus as set forth in claim 1 in which: cam-forming means are provided on those opposed surfaces of the tangs or base bordering the slot therein that first engage one another upon rotational movement of the ring means from its released into its locked position, said cam-forming means being effective to deflect said tangs over onto the planar surface.

9. The apparatus as set forth in claim 8 in which: the cam-forming means comprise beveled leading edges in the tangs.

10. The apparatus as set forth in claim 8 in which: the cam-forming means comprise oppositely-inclined areas separating the roof-forming surface at the top of the socket from the margins of the slot.

lll. The front riflescope mounting subassembly for use with a base having a downwardly and inwardly tapered seat-forming frusto-conical depression in its top surface, a socket in the underside thereof opposite said depression in the top terminating at its upper end in a roof-forming surface. and an elongate essentiallydia metrical slot intersecting said depression communicating the latter with the socket therebeneath which comprises: ring means having a post depending from the underside thereof terminating at its lower end in a downwardly and inwardly tapered frusto-conical tip section complementary to the seat-forming depression in the base, and an elongate clip sized to pass through the slot in the base depending from the apex of the truncated tip section, said clip defining a latch member effective to hold said ring means in assembled relation atop the base when inserted through the slot in the latter and rotated relative thereto into a position underlying the roof-forming portion at the top of the socket.

12. The subassembly as set forth in claim 11 in which: the clip includes a pair of springable tangs projecting in opposite directions from its point of attachment to the apex of the frusto-conical tip section, said tangs being operative in assembled relation to engage the roofforming surface at the top of the socket and cooperate therewith to continually bias the complementary frusto-conical surfaces into mated slidable engagement with one another.

13. The subassembly as set forth in claim 11 in which: the frusto-conical tip section of the post is a right conical section having its axis passing through the center of the ring means.

14. The subassembly as set forth in claim 11 in which:

the frusto-conical tip section is larger at its base than the complementary' frusto-conical seat-forming surface so as to extend above the latter when in assembled relation and define a stop effective to keep the post from contacting the base.

15. The subassembly as set forth in claim 11 in which: the upper base-engaging surface of the tangs is essentially planar and normal to the axis of ring rotation.

16. The base for detachably connecting a riflescope mounting ring to a rifle which comprises: a rigid platform having the underside thereof shaped to conform with the rifle barrel atop which it is to be mounted, a frusto-conical seat-forming depression in the upper surface thereof shaped to mate with a complementary frusto-conical post tip on the underside of the ring, a socket in the underside directly beneath the frustoconical depression, and an elongate slot extending diametrically across said depression communicating the latter with the socket therebeneath, said socket and slot cooperating to receive and releasably retain a transversely-extending latch member depending from the apex of the post upon insertion of the latter through the slot and relative rotation through an angle sufficient to establish a misaligned relation therebetween.

17. The base as set forth in claim 16 in which: the frusto-conical depression is a right conical surface.

18. The base as set forth in claim 16 in which: the socket terminates at its upper end in a roof-forming surface that is essentially planar and normal to the axis of the frustoconical depression thereabove. 

1. Apparatus for detachably fastening the front end of a riflescope to a rifle so as to permit limited relative pivotal movement of the former relative to the latter which comprises: a rigid base fixedly attachable to the rifle having a downwardly and inwardly tapered seat-forming frusto-conical depression in the top surface thereof, a socket in the underside thereof opposite said depression in the top terminating at its upper end in a roof-forming surface, and an elongate essentially diametrical slot intersecting said depression establishing communication between the latter and the socket therebeneath; ring means having a post depending from the underside thereof terminating at its lower end in a downwardly and inwardly tapered frusto-conical tip section complementary to the seat-forming depression in the base; and, latch-forming means shaped to enter the slot in the base when aligned therewith operative upon limited relative rotation between said base and ring to releasably lock same together in assembled relation, said means comprising a clip attached to the truncated apex of the tip section having a pair of springable tangs projecting in opposite directions from said point of attachment, said tangs being spaced beneath the frusto-conical section thereabove a distance such that they will seat against the roof-forming surface when rotated into face-to-face contacting relation therewith, and said tangs and roof-forming surface cooperating with one another in assembled relation to continually bias said opposed frustoconical surfaces into tight slidable mated engagement.
 2. The apparatus as set forth in claim 1 in which: the roof-forming surface at the top of the socket is essentially planar and perpendicular to the axis of ring rotation.
 3. The apparatus as set forth in claim 1 in which: the frusto-conical tip section of the post is a right conical section having its axis passing through the center of the ring means.
 4. The apparatus as set forth in claim 1 in which: the complementary frusto-conical surfaces are both right conical sections; and, in which the tangs and roof-forming surface cooperate with one another in assembled relation to maintain a coaxial relationship between the respective axes of said complementary frusto-conical surfaces.
 5. The apparatus as set forth in claim 1 in which: the frusto-conical tip section is larger at its base than the complementary frusto-conical seat-forming surface so as to extend above the latter when in assembled relation and define a stop effective to keep the post from contacting the base.
 6. The apparatus as set forth in claim 1 in which: the upper base engaging surface of the tangs is essentially planar and normal to the axis of ring rotation.
 7. The apparatus as set forth in claim 1 in which: downwardly facing shoulders define the margin separating the frusto-conical tip section from the post thereabove; and, in which the mated frusto-conical surfaces cooperate with one another in assembled relation to hold said shoulders spaced above the top of the base.
 8. The apparatus as set forth in claim 1 in which: cam-forming means are provided on those opposed surfaces of the tangs or base bordering the slot therein that first engage one another upon rotational movement of the ring means from its released into its locked position, said cam-forming means being effective to deflect said tangs over onto the planar surface.
 9. The apparatus as set forth in claim 8 in which: the cam-forming means comprise beveled leading edges in the tangs.
 10. The apparatus as set forth in claim 8 in which: the cam-forming means comprise oppositely-inclined areas separating the roof-forming surface at the top of the socket from the margins of the slot.
 11. The front riflescope mounting subassembly for use with a base having a downwardly and inwardly tapered seat-forming frusto-conical depression in its top surface, a socket in the underside thereof opposite said depression in the top terminating at its upper end in a roof-forming surface, and an elongate essentially diametrical slot intersecting said depression communicating the latter with the socket therebeneath which comprises: ring means having a post depending from the underside thereof terminating at its lower end in a downwardly and inwardly tapered frusto-conical tip section complementary to the seat-forming depression in the base, and an elongate clip sized to pass through the slot in the base depending from the apex of the truncated tip section, said clip defining a latch member effective to hold said ring means in assembled relation atop the base when inserted through the slot in the latter and rotated relative thereto into a position underlying the roof-forming portion at the top of the socket.
 12. The subassembly as set forth in claim 11 in which: the clip includes a pair of springable tangs projecting in opposite directions from its point of attachment to the apex of the frusto-conical tip section, said tangs being operative in assembled relation to engage the roof-forming surface at the top of the socket and cooperate therewith to continually bias the complementary frusto-conical surfaces into mated slidable engagement with one another.
 13. The subassembly as set forth in claim 11 in which: the frusto-conical tip section of the post is a right conical section having its axis passing through the center of the ring means.
 14. The subassembly as set forth in claim 11 in which: the frusto-conical tip section is larger at its base than the complementary frusto-conical seat-forming surface so as to extend above the latter when in aSsembled relation and define a stop effective to keep the post from contacting the base.
 15. The subassembly as set forth in claim 11 in which: the upper base-engaging surface of the tangs is essentially planar and normal to the axis of ring rotation.
 16. The base for detachably connecting a riflescope mounting ring to a rifle which comprises: a rigid platform having the underside thereof shaped to conform with the rifle barrel atop which it is to be mounted, a frusto-conical seat-forming depression in the upper surface thereof shaped to mate with a complementary frusto-conical post tip on the underside of the ring, a socket in the underside directly beneath the frusto-conical depression, and an elongate slot extending diametrically across said depression communicating the latter with the socket therebeneath, said socket and slot cooperating to receive and releasably retain a transversely-extending latch member depending from the apex of the post upon insertion of the latter through the slot and relative rotation through an angle sufficient to establish a misaligned relation therebetween.
 17. The base as set forth in claim 16 in which: the frusto-conical depression is a right conical surface.
 18. The base as set forth in claim 16 in which: the socket terminates at its upper end in a roof-forming surface that is essentially planar and normal to the axis of the frustoconical depression thereabove. 